Rotary target for electrostatic accelerator which operates as a neutron generator

ABSTRACT

The rotary target has a generally annular shape and is carried by a rotor into which the beam of the accelerator penetrates, a stator being placed around said rotor and forming an end-shield behind the target. Heat-transporting fluid is circulated within the space formed between the end-shield and the target as a result of rotation of the rotor which acts on the fluid in the same manner as a pump. A heat exchanger is placed on the path of the heat-transporting fluid and at least one system of baffles is placed between the rotor and the stator.

United States, Patent [191 Roche 1 May 15, 1973 [54] ROTARY TARGET FOR ELECTROSTATIC ACCELERATOR WHICH OPERATES AS A NEUTRON GENERATOR [75] Inventor: Michel Roche, Quetigny, France Attorney ca'mgron, Kerkam & Sutton [73] Assignee: Commissariat A LEnergie Atomique, Paris, France 221 Filed: Jan. 8, 1971 7 ABSTRACT [211 APPI- 104,951 "rire'raiarytargzms'ygemzawr imam; 30] Foreign A ifli fi priority Data carried by a rotor into which the beam of the accelerator penetrates, a stator being placed around said rotor and forming an end-shield behind the target. Jan. 20, 1970 France ..7001864 Heat-transporting fluid is circulated within the space Sept.9,1970 France ..7034668 formed between the end-shield and the target as a result of rotation of the rotor which acts on the fluid [52] US. Cl. ..250/84.5, 313/61, 313/330 in the same manner as a pump. A heat exchanger is [51] Int. Cl. ..G2lg 3/00 placed on the path of the heat-transporting fluid and [58] Field of Search ..250/84.5; 313/60, at least one system of baffles is placed between the 313/61, 330, 11 rotor and the stator.

[56] References Cited A '7 V V I 5 Claims, 3 Drawing Figures UNITED STATES PATENTS 33311978 WES? 121015175 aTIIQ.llllLlQLILI?)13/330' 3,609,366 9/1911" Croitoru .izii/ims Primary Examiner-David Schonber'g Assistant Examiner-Robert L. Sherman M PATENTEU m 1 5191s sum 1 or 3 PA TENTED RAY I 5 I975 SHEET 2 0F 3 ROTARY TARGET FOR ELECTROSTATIC ACCELERATOR WHICH OPERATES AS A NEUTRON GENERATOR This invention relates to a rotary target for an electrostatic accelerator which operates as a'neutron generator.

The invention is more particularly directed to a proposal for solving the problem of removal of the power which is dissipated on the target, this problem being more and more complex as higher power levels are progressively attained by accelerators of this type if consideration is given to the following essential requirements:

in order to have good accessibility for the emission of a high neutron flux, it must be possible to place a sample in very close proximity to the active surface of the target;

in order to employ the target under good conditions, the speed of rotation of the target must be as high as possible;

finally, in order to ensure good operation of the accelerator, there must not be any detectable vacuum contamination, especially by hydrocarbons.

The primary object of the present invention is to improve the targets aforesaid in such a manner as to meet practical requirements more effectively than has been the case up to the present time, especially insofar as they provide a solution to the problem of maximum removal of the power which is dissipated on the target and in the beam impact surface while at the same time maintaining a target temperature which is as high as possible.

In accordance with the invention, a rotary target for an electrostatic accelerator which operates as a neutron generator is characterized in that the target proper has a generally annular shape and is carried by a rotor into which the beam of the accelerator penetrates, said rotor being provided with a shaft which is carried by at least one bearing and is coupled for rotation by any suitable means, a stator being placed around said rotor and forming an end-shield behind said target, and a circulation of heat-transporting fluid being provided within the space formed between said end-shield and said target, said circulation being carried out as a result of rotation of the rotor which acts on said fluid in the same manner as a pump and provision being additionally made on the path of said fluid for a heat exchanger and for at least one system of baffles which are placed between said rotor and said stator.

In a preferred embodiment of the invention, said rotor has a frustoconical shape, the beam being intended to penetrate through the summit of said cone and the annular target being placed at the base thereof while the lateral surface of said rotor is provided with shoulders forming low-conductance baffles, said cone being connected to said shaft both by means of fins and by means of said target, said shaft being carried by two ball bearings and being coupled for rotation by means of a pulley, said stator being provided with a plurality of cavities and differential pumping being carried out in each cavity, one wall of each cavity being maintained at the temperature of liquid nitrogen, leak-tightness between said end-shield and said shaft being maintained by means of a lipped seal, a fluid having a relatively high vapor pressure and especially water being employed as heat-transporting fluid and cooled within a heat exchanger prior to being reinjected between said end-shield and said target, said target being preferably formed of thin sheet copper.

In another preferred embodiment of the invention, said rotor has a flat shape, said shaft is driven by a suitable magnetic device and said heat-transporting fluid is a fluid having a low vapor pressure and cooled in a water exchanger, an annular liquid-nitrogen trap being provided is particular for pumping the vapor of said heat-transporting fluid.

In the embodiment last mentioned, said shaft can be supported by a magnetic bearing or a fluid bearing and in the latter case the fluid of the bearing is the heattransporting fluid and especially sodium or mercury.

In accordance with improvements made in said second embodiment, the rotary target for an electrostatic accelerator which operates as a neutron generator is characterized in that said baffles are designed in the form of at least two coaxial toric cavities which are formed in said stator at the level of a stepped section provided in the vicinity of the circumference of said rotor of flat circular shape, said cavities being open on the portion which is adjacent to said rotor, said openings having generally circular shapes limited by sharp edges of said stator, said sharp edges being located in the immediate vicinity of said rotor.

In an even more specific embodiment, provision is made for two toric cavities and four circular sharp edges which define from the interior towards the exterior successive portions of the stator surface which is located opposite to the rotor so that a surface which is parallel to that of the rotor is located before the first sharp edge, a conical surface which comes closer to the rotor is located between the second and the third sharp edge, a surface which is parallel to the rotor surface is located after thefourth sharp edge, said stepped section of the rotor being located at the level of the third sharp edge.

The complementary description which now follows and the accompanying drawings will in any case serve to gain a better understanding of the invention and are given primarily by way of indication without any limitation being implied.

Reference is made to the drawings, wherein:

FIG. 1 is an axial sectional view of a first embodiment of a rotary target in accordance with the invention;

FIG. 2 is an axial sectional view of a second embodiment of a rotary target in accordance with the invention;

FIG. 3 is an axial sectional view of an improved alternative forrn of execution of the embodiment of FIG. 2.

In the embodiment which is illustrated in FIG, 1, provision is made for a target which is cooled by a fluid having a relatively high vapor pressure such as water, for example.

A rotor 1 has the general shape of a cone frustum into which the beam 3 penetrates through the summit 5 while the base 7 of said cone frustum has an annular surface 9 which forms the target proper. The lateral surface of the cone frustum is provided with a plurality of shoulders such as those designated by the reference numeral 11, the design function of said shoulders being to provide low-conductance baffles. The rotor unit is carried by a shaft 13 and joined to this latter by means of fins such as the fin 15 and also by means of the base 7 and the target 9. The leading edge of each fin such as is protected from the beam 3 by a heat-insulated tungsten rod 17.

The shaft 13 is supported by two ball bearings 19 and 21 and can be driven in rotation by means of a pulley 23.

The stator 25 has a plurality of cavities such as the cavity 27 and differential pumping is carried out within each of these latter, with the result that a high degree of leak-tightness can be obtained between the rotor 1 and the stator 25. Within each cavity 27, one wall is maintained at the temperature of the liquid nitrogen in order to ensure effective pumping of the steam. The stator 25 is connected by means of a protected opening 29 to the vacuum passage through which the beam 3 arrives and to an end-shield 31 which is located behind the target 9 and serves to convey the coolant water behind said target 9 in a very thin film of the order of 0.5 mm. Leak-tightness between the end-shield 31 and the shaft 13 is ensured by means of a lipped seal.

A baffle 33 prevents the water which is centrifuged during operation from flowing in the liquid state into the pumping baffles such as the baffle 11; Furthermore, the baffle 33 has low steam conductance.

The operation of the target which is illustrated in FIG. 1 will now be described.

During rotation, the rotor 1 behaves in the same manner as a pump for the coolant water which is circulated behind the target 9 at very high velocity. Since the target is preferably formed of thin copper sheet having a thickness of 0.5 to 1 mm, the cooling action of the water is very powerful. The water is then centrifuged to the external portion, flows out through the duct 35 and is cooled within a heat exchanger 37, then reinjected at 39 into the space formed between the end-shield 31 and the target 9. Evaporation of the water takes place within the baffle 33 and the pressure is equal to the steam pressure, that is to say approximately 15 torr at 30 C. The pumping operations which are carried out within each cavity 27 will then permit of progressive reduction in pressure so that the quantity of water introduced into the accelerator chamber will be negligible.

The advantage which is offered by the embodiment described with reference to FIG. 1 is apparent: on the one hand, the differential-pumping rotary seal operates with an upstream pressure which is lower than atmospheric pressure and, on the other hand, a gas which can readily be pumped is injected into said sea].

In the embodiment which is illustrated in FIG. 2, provision is made for a target which is cooled by a fluid having a low vapor pressure such as sodium or mercury, for example, that is to say a vapor pressure of the order of 10' torr.

This principle is the same as in the case of FIG. 1 but the structural arrangement is highly simplified since the rotary seal can be reduced to a single liquid-nitrogen trap which forms a baffle. In order to eliminate the lipped seal and thus to permit the attainment of very high speeds of rotation of the order of 4,000 rpm, for example, while at the same time increasing the quality of the vacuum, the rotor 51 which is of flat shape in the example shown can be supported by a fluid bearing 53 and rotated by means of a magnetic drive unit 55. The rotor 51 could also be supported by a magnetic hearing or by special sodium-lubricated ball bearings but in the case of a fluid bearing the bearing-fluid can be the heattransporting fluid itself. The target proper is constituted by an annular portion 57 of the rotor 51, said portion 57 being located opposite to the inlet of the beam 59. A few baffles such as the baffle 61 are provided on the external portion of the rotor 51 and cooperate with baffles such as 63 which are carried by the stator 65 in order to prevent the coolant from penetrating into the vacuum chamber in the liquid state. The coolant-fluid vapor is pumped by means of an annular liquidnitrogen trap 67 which is supplied through the pipe 69. A water exchanger 71 serves to cool said fluid. The rotor 51 performs the function of a pump for circulating the heat-transporting fluid in the same manner as the rotor 1 of FIG. 1. Finally, in the event that the shaft 73 of the rotor 51 is carried by a fluid bearing 53, the rotor 51 which supplies the fluid bearing 53 can be started by means of two retractable ball bearings (not shown in the drawings).

When sodium is employed as coolant, the device which is illustrated in FIG. 2 permits the achievement of particularly advantageous performances:

total power dissipation higher than kW,

dissipation in the beam impact surface l0 kW/cm in respect of a target temperature which is lower than 250 C.

The embodiment which will now be described with reference to FIG. 3 is of the type comprising in the case of the arrangement shown in FIG. 2 a rotor which has a flat shape and a shaft which is driven by a suitable magnetic device. Cooling is carried out by a heat transporting fluid having a low vapor pressure and cooled in a water exchanger, provision being made for an annular liquid-nitrogen trap which serves to pump the vapor of said heat-transporting fluid. In this embodiment, the heat-transporting fluid can be either sodium or mercury.

In FIG. 3, identical reference numerals are assigned to elements which are similar to the corresponding elements of FIG. 2 while modified elements bear the same reference numeral followed by the letter a.

As can be seen from this figure, provision has also been made for a target which is cooled by a fluid having a low vapor pressure, that is to say of the order of 10 torr, said fluid being either sodium or mercury, for example.

The rotor 51 has a flat shape and is carried by two ball bearings 19 and 21, said rotor being driven in rotation by means of a magnetic drive unit 55. The target proper is constituted by an annular portion 57 of the rotor 51 which is located opposite to the inlet of the beam 59. The rotor 51 is provided in the vicinity of its external portion with a stepped section 61a which performs the function of the baffles 61 of FIG. 2 and cooperates with baffles such as 63a which are provided on the stator 65 in order to prevent the coolant fluid from penetrating into the vacuum chamber in the liquid state.

In the form of construction which is illustrated, the baffles 63a are formed by two coaxial toric cavities 81 and 83 which are arranged within the stator 65 opposite to the stepped section 61a. Said cavities 81 and 83 are open on that surface of the stator 65 which is located in oppositely-facing relation to the surface of the rotor 51 and define four sharp circular edges 85, 87, 89 and 91 respectively from the interior towards the exterior of the device. Up to the level of the sharp edge 85, the stator surface 93 is parallel to that of the rotor and close to this latter. Between the sharp edges 87 and 89,

the surface 95 of the stator is conical and comes closer to that of the rotor. After the sharp edge 91, the surface 97 of the stator is parallel to that of the rotor and close to this latter. Finally, it is apparent that the stepped section 61a of the rotor is located opposite to the sharp edge 89.

As in the embodiment of FIG. 2, the coolant-fluid vapor is pumped by an annular liquid-nitrogen trap 67. Similarly, the rotor 51 performs the function of a pump for circulating the heat-transporting fluid.

What we claim is:

1. A rotary target for an electrostatic accelerator neutron generator subject to impact of a beam of accelerated particles, comprising an annular target, a rotor supporting said target, the beam of the accelerator entering said rotor, a shaft for said rotor, at least one bearing for said shaft, means for rotating said shaft, a stator under vacuum around said rotor including an end-shield spaced behind said target, means for circulation of a heat-transporting fluid in said stator under vacuum within the space between said end-shield and said target including said rotor pumping said fluid, a heat exchanger for said fluid and at least one system of baffles between said rotor and said stator.

2. A rotary target according to claim 1, said rotor having a frustoconical shape, the-beam of particles entering the summit of said cone, said target being at the base of said cone, a lateral surface for said rotor, shoulders on said surface forming low-conductance baffles, fins connecting said cone to said shaft, said target connecting said cone to said shaft, a pulley on said shaft, a plurality of cavities in said stator receiving said shoulders for differential pumping in each of said cavities, one wall of each of said cavities being maintained at the temperature of liquid nitrogen, and a lipped seal between said end-shield and said shaft.

3. A rotary target according to claim 1, said rotor being flat, a magnetic device rotating said shaft and said heat-transporting fluid having a low vapor pressure.

4. A rotary target according to claim 1, said baffles including at least two coaxial toric cavities in said stator, a stepped section at the circumference of said rotor opposite said cavities, openings for said cavities opening toward said rotor, said openings being generally of circular shape, and sharp edges for said openings, said sharp edges being in the immediate vicinity of said rotor.

5. A rotary target according to claim 4, including two of said toric cavities and four of said circular sharp edges defining from the interior towards the exterior successive portions of the stator surface opposite said rotor, one of said portions being parallel to said rotor and before the first of said sharp edges, a second of said portions being a conical surface closer to said rotor and between the second and the third sharp edge, a third of said portions being a surface parallel to said rotor surface and after the fourth sharp edge, said stepped section of said rotor being opposite the third sharp edge. 

1. A rotary target for an electrostatic accelerator neutron generator subject to impact of a beam of accelerated particles, comprising an annular target, a rotor supporting said target, the beam of the accelerator entering said rotor, a shaft for said rotor, at least one bearing for said shaft, means for rotating said shaft, a stator under vacuum around said rotor including an end-shield spaced behind said target, means for circulation of a heat-transporting fluid in said stator under vacuum within the space between said end-shield and said target including said rotor pumping said fluid, a heat exchanger for said fluid and at least one system of baffles between said rotor and said stator.
 2. A rotary target according to claim 1, said rotor having a frustoconical shape, the beam of particles entering the summit of said cone, said target being at the base of said cone, a lateral surface for said rotor, shoulders on said surface forming low-conductance baffles, fins connecting said cone to said shaft, said target connecting said cone to said shaft, a pulley on said shaft, a plurality of cavities in said stator receiving said shoulders for differential pumping in each of said cavities, one wall of each of said cavities being maintained at the temperature of liquid nitrogen, and a lipped seal between said end-shield and said shaft.
 3. A rotary target according to claim 1, said rotor being flat, a magnetic device rotating said shaft and said heat-transporting fluid having a low vapor pressure.
 4. A rotary target according to claim 1, said baffles including at least two coaxial toric cavities in said stator, a stepped section at the circumference of said rotor opposite said cavities, openings for said cavities opening toward said rotor, said openings being generally of circular shape, and sharp edges for said openings, said sharp edges being in the immediate vicinity of said rotor.
 5. A rotary target according to claim 4, including two of said toric cavities and four of said circular sharp edges defining from the interior towards the exterior successive portions of the stator surface opposite said rotor, one of said portions being parallel to said rotor and before the first of said sharp edges, a second of said portions being a conical surface closer to said rotor and between the second and the third sharp edge, a third of said portions being a surface parallel to said rotor surface and after the fourth sharp edge, said stepped section of said rotor being opposite the third sharp edge. 